This disclosure relates generally to electromagnetic downhole tools and, more particularly, to electromagnetic downhole tools having multiple transmitters to compensate for effects of environmental conditions on receiver circuitry.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Corrosion of wellbore casing is a natural phenomenon in the harsh environment of oil and gas production wells. In fact, such corrosion may be extremely costly, potentially exceeding billions of dollars. The casing therefore may be monitored to enable timely mitigation strategies to prevent leaks, environmental damages, and other failures. Various downhole tools have been developed to detect corrosion in well casing. In one example, pipe corrosion can be inferred from the measurement of the internal diameter and/or wall thickness of a pipe in a well. Mechanical calibers can provide the internal diameter. Ultrasonic tools can measure both the internal diameter and the thickness of a fluid-filled pipe. Electromagnetic (EM) tools can evaluate corrosion in single- and multiple-casing wells by determining metal loss and inner casing geometry based on the behavior of electromagnetic signals launched and received from transmitters and receivers located in an innermost tubing.
Considering electromagnetic (EM) tools in particular, these tools may use transmitter coils to excite eddy currents in the casing. The eddy currents in the casing generate corresponding magnetic fields. Receiver coils may measure the effect of the casing on the eddy currents to obtain certain properties of the casing. For example, the receiver coils may obtain near field measurements when a closely spaced transmitter coil excites high-frequency eddy currents to determine a transimpedance value, or Z-property, that corresponds to the internal diameter (ID) of the casing. Variations in the Z-property of the casing may indicate possible corrosion.
The receiver coils that obtain these measurements, however, may be susceptible to the harsh environmental conditions of the well that is being measured. Among other things, the receiver coils may obtain different measurements depending on both the magnitude of the current temperature, as well as a hysteresis of recent temperature changes. Although some techniques have been developed to calibrate the receiver coils to account for thermal drift, these calibrations may be very complex and, at times, may be less accurate than desired. Such calibrations may also be difficult because the thermally dependent components may be located in different regions of a downhole tool, each with its own thermal environment. The complexity of the calibrations may be compounded by relying on a thermal reference, which may be located in still a different thermal area. As a result, the ultimate Z-property measurements may be less accurate and/or less precise than desired.